A simple method of recording heart sounds and murmurs

Methods

A Simple Method of Recording Heart Sounds and Murmurs

DENIS ABELSON, MD DAVID BERNBAUM Philadelphia,

Pennsylvania

A new technique (frequency phonocardiography) is described for the graphic registration of heart sounds and murmurs. With use of a zero-crossing detector, an analog voltage is developed proportional to frequency. Advantages of the method include clarity of presentation, ease of measurement of time intervals and suitability for mechanical recorders.

Conventional phonocardiograms are useful for displaying the temporal relations of heart sounds and murmurs and their relative amplitudes. For faithful reproduction the response of the recorder should be at least as high as 600 Hz, although by inscribing only the envelope of sound amplitude a mechanical recorder with more restricted high frequency response can be used.le2 In the analysis of frequency components more complex methods are employed.3-10 This communication describes a simple technique for recording the frequency of sounds and murmurs, which is suitable for direct-writing machines such as electrocardiographs. The record so obtained may conveniently be termed a frequency phonocardiogram.

Method

From the Department of Medicine, the Graduate Hospital, University of Pennsylvania, Philadelphia, Pa. Manuscript received April 6, 1970, revised manuscript received August 24, 1970, accepted October 2, 1970. Address for reprints: Denis Abelson, MD, Clinical Research Unit, the Lankenau Hospital, Lancaster and City Line Ave., Philadelphia, Pa. 19151.

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A heart sound microphone (Fig. 1) is connected to an amplifier (heart-sound amplifier, Cambridge Instrument Co.) (response d-c to 1,000 HZ), the output from which is fed to a zero-crossing detector (velocity-amplitude converter, Smith, Kline Instrument Co.). The latter device, which is normally used in conjunction with an ultrasonic Doppler detector for the measurement of blood flow velocity, provides an analog voltage proportional to the dominant frequency. The output of the zerocrossing detector can be connected directly to an electrocardiograph or, after suitable amplification and filtering, to an oscilloscopic or mechanical recorder. The present illustrations were made with a multichannel oscilloscopic recorder (MC-IV multichannel recorder, Cambridge Instrument Co.) in order to correlate data from the frequency phonocardiogram with other data including those from the original phonocardiogram. For portable use we have adapted a 2 channel direct-writing (thermal) recorder (Fig. 2)) which permits simultaneous comparison with the electrocardiogram.* Calibration of the frequency phonocardiogram is carried out either with a push-button delivering a standard voltage (100 mv = 1 kHz), or by a tuning fork (for example, 128 Hz), placed in front of the microphone suspended in the air. Both contact and air-coupled microphones have been used successfully. The audio amplifier used had a minimal gain of 2,000 and a maximal analog output of 4 volts peak to peak, It was provided with 2 optional filters, a Leathem-type filter for rejecting low frequencies and a band-pass filter adjustable from 70 to 700 Hz. These filters permit frequencies of interest to be displayed preferentially. The zero-crossing detector possessed the following characteristics: * An instrument based on the prototype ments, Inc., Bala Cynwyd, Pa. 19004.

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ELECTROCARDIOGRAPH

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Figure 1. Block diagram of the frequency phonocardiograph.

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Figure 2. Frequency phonocardiograph and elect1ocardiographic recorder. Prototype shejwing (left to right) dual-channel therm al recorder, general purpose amplifier, electrocardiographic amplifier, heart sound amplifier and velocity converter. mV

Figure 3. Behavior of the zero-crossing detector when 2 signals (Fl, F2) of different frequency and nearly equal amplitude are presented to it. Bottom tracing, mixed input to the detector; upper tracing, output. Fl = 500 Hz at 0.2 v peak to peak; F2 = 450 Hz at 0.2 v peak to peak.

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FREQUENCY

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Simultaneous recordings from a patient with mitral incompetence and Figure 4. unfiltered), frequency recording atrial fibrillation. A, phonocardiogram (apex, (FPCG), carotid pulse and electrocardiogram (lead II). B, phonocardiogram, frequency phonocardiogram, right common carotid arterial flow (recorded with detector) and electrocardiogram. The longer Doppler probell and zero-crossing R-R intervals are followed by increases in carotid pulse amplitude and flow velocity, and in the amplitude and frequency of the systolic murmur. Paper speed 7.5 cm/set.

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Mitral stenosis and insufficiency, with atrial fibrillation. Systolic murmur Figure 5. and opening snap. Upper tracing, phonocardiogram from left sternal border, bandpass filter set at 70 Hz. Middle tracing, frequency phonocard4ogram. Lower tracing, electrocardiogram (lead II). Paper speed 10 cm/set.

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1. output: This was adjusted so that 1,000 Hz produced 100 mv d-c. 2. Detector threshold: The “reject” control was adjusted to a point in amplitude between background noise and minimal desired signal. With use of a 250 Hz signal input from a sine-wave generator, it was found that 26 mv was required to reach the amplitude threshold. At 10 times this voltage the threshold was crossed at 33.8 Hz, and the frequency response was linear from 40 to 3,000 Hz.

TABLE

This was found to be 12.8 msec, 3. Response time: using a sine-wave generator switched from 300 to 3,000 Hz (low frequency rejection set at 26 mv as for the detector threshold). 4. Interference rejection: A test was carried out of the ability of the detector to discriminate between 2 signals of different frequency. Two signal geherators were connected by 10 kilo-ohm resistors to the input of the detector. Two pairs of frequencies were tested, 1 pair close together (450 and 500 Hz) and 1

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Response of the Zero-Crossing Detector to 2 Signals of Different Frequency, Presented Simultaneously

Frequency (Hz) of Signal of Fixed Amplitude (0.4 v peak to peak) Pair 1

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* With one signal fixed, the amplitude of the other is increased until a “zone of confusion” is reached. In this region the detector spends some time at one voltage and some time at the other, for example, 50 and 45 mv for Pair 1 (see Fig. 3). Al, A2 = voltages of the low and high amplitude signals, respectively, at the limit of the zone of confusion.

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Figure 6. Fourth heart sound in congestive heart failure and left bundle branch block. Upper tracing, phonocardiogram, apex. Middle tracing, electrocardiogram (lead II). Lower tracing, frequency phonocardiogram. Paper speed 10 cm/set.

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Unusually high pitched, soft diastolic murmur in aortic insufficiency with congestive heart failure. A, phonoFigure 7. cardiogram (700 Hz) from left sternal border, and electrocardiogram (lead II). B, frequency phonocardiogram and electrocardiogram. The upper calibration line corresponds to 1,000 Hz. The first and second heart sounds, being low pitched, are largely filtered out, leaving the high pitched diastolic murmur clearly visible. Paper speed 10 cm/set.

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Figure 8. Systolic murmur (S.M.), third and fourth heart sounds in angina pectoris, with right bundle branch block and 2 previous ‘infarctions. The patient was in the left lateral prone position. Upper tracing, electrocardiogratm (lead II). Middle tracing phonocardiogram (70 HZ), apex. Lower tracing, frequency phonocardiogram. For improved registration of the lower frequencies a contact microphone (Electronics for Medicine, Inc.) was used, with its attached filter set to the “apex” position. Paper speed 10 cm/set.

further apart (100 and 500 Hz). In each case the amplitude of one signal generator ( (Fl) was fixed at 0.4 v; that of the other (F2) was gradually increased from 0. We noted the level of voltage of F2 at which the detector output became unsteady, ceasing to provide a reliable estimate of the frequency of Fl. The voltage of F2 was then further increased, and a note was made of the point at which the detector output gave a steady indication of the frequency of F2. Results are shown in Table I. It can be seen that signals as close together as 450 Hz and 500 Hz are not distinguishable if they differ in amplitude by lees than an octave (Fig. 3). It also appears that, for a given amplitude, a signal with a frequency lower than that of the signal of interest is less likely to interfere than a signal with a higher frequency.

Illustrative Tracings Figure 4 shows the effect of variations in the length of the R-R interval in a patient with mitral insufficiency and atria1 fibrillation. With long intervals the carotid pulse amplitude is larger and the velocity of carotid blood flow is augmented, in accordance with Starling’s law. At the same time, the mid-systolic murmur increases in amplitude and frequency, although in relation to the heart sounds the latter change is more striking. The frequency phonocardiogram also shows some beat to beat variation in the first and second heart sounds, which are frequently ‘split.

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Figures 5 and 6 show, respectively, recordings of an opening snap and a fourth heart sound. Although relatively inconspicuous in the phonocardiogram, they are convincingly demonstrated in the frequency phonocardiogram. Figure ‘7 illustrates the difficulty of making a satisfactory conventional phonocardiogram of a high pitched murmur of low amplitude. By contrast, the frequency phonocardiogram shows the murmur clearly.

Advantages of Method The method described does not provide a thorough frequency analysis, but merely indicates the dominant frequency. Nonetheless, its simplicity should permit more widespread study of the frequency characteristics of different murmurs, and of day to day and beat to beat changes in frequency. Some sounds and murmurs can be more clearly demonstrated with the frequency method than with conventional techniques. Furthermore, time intervals, such as that between the second heart sound and the opening snap (Fig. 5) or third heart sound (Fig. 8), can readily be measured by this method.

Acknowledgment Our thanks are due to Mr. Peter Lombardo, who constructed the prototype recorder.

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References 1. Rushmer 2. 3.

4. 5. 6.

RF, Bark RS, Ellis RM: Direct-writing heartsound recorder. Amer J Dis Child 83:733-739, 1952 Ruohmer RF, Sparkman DR, Polley RFL, et al: Variability in detection and interpretation of heart murmurs. Amer J Dis Child 83:740-754, 1952 McKusick VA, Talbot SA, Webb GN: Spectral phonocardiography. Problems and prospects in the application of the Bell sound spectrograph to phonocardiography. Bull Johns Hopkins Hosp 94:187-198.1954 McKusick VA. Webb GN. Bravshaw JR. et al: Soectral phonocardiography: clinical s&dies. Bull Johns H’opkins Hosp 95:90-110, 1954 Koenig W, Ruppel AE: Quantitative amplitude representation in sound spectrograms (Bell Labs monograph B-1622). J Acoust Sot Amer 20:787-795,1948 Prestigiacomo AJ: Amplitude contour display of sound

spectrograms (Bell Labs monograph 4388). J Acoust Sot Amer 34:168&1688, 1962 Winer DE, Perry LW, Caceres CA: Heart sound analysis: a three dimensional approach. Amer J Cardiol 16:547551, 1965 Rogers WM, Harrison JS, Blanchard S: Contour spectral phonocardiography in the assessment of aortic valve disease. Ann NY Acad Sci 147:690-715,1969 Huggins WH: A phase principle for complex frequency analysis and its implications in auditory theory. J Acoust Sot Amer 24:582-589, 1952 10. van Vollenhoven E, van Rotterdam A, Dorenbos T, et al: Frequency analysis of heart murmurs. Med Biol Engin 7:227-232, 1969 Ultrasonic Doppler auscultation of the 11. Abelson DA: heart, with observations on atrial flutter and fibrillation. JAMA 204:438443,1968

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